Butterfly valves are particularly desirable in a number of applications because of their rapid operation and their relatively high flow capacity. However, to be completely leakproof, especially at high pressures, the valve member must be very accurately positioned with respect to the seal against which it seats. This has required close tolerance machining of not only the valve itself but also of the stem or shaft which mounts the valve and the bearings mounting the shaft. Because these valve housings are sand cast they cannot be cast to the accuracy required for effective operation using conventional valve designs and manufacturing methods.
Because of the access problems inherent in butterfly valve design the tooling for performing the necessary machining is expensive. The fact that machining the valve stem opening necessitates extending the tool from one side of the valve body to the other creates serious difficulty in maintaining accurate alignment, an essential for effective valve operation. If any amount of cut has to be eccentric, this will, unless great care is exercised, cause the tool to wander, resulting in misalignment. Even with expensive and properly operated tooling it is extremely difficulty to consistently maintain accuracy within the tolerances necessary to assure leakproof, high pressure operation of the valves. Further, the machining is very expensive not only because it is demanding of equipment but it is slow and requires very skilled operators. Particularly is this true in the boring and reaming of the valve body to receive the valve stem.
Another problem is that of the high torque required to operate the conventional butterfly valve. If the valve is not equipped with bearings for the valve stem, even the best machining normally results in a valve having excessive torque due to friction between the valve stem and the housing. The addition of bearings normally reduces this high torque, but only to a degree, because of the great difficulty of maintaining highly accurate alignment.
Another factor contributing to high closing torque characteristic of conventional butterfly valves is the amount of compression of the seal by the vane or valve member. The primary area in which leakage is experienced is in the area adjacent the stem. Therefore, a high degree of compression is required in that area. However, further away from the axis of rotation of the valve less seal compression is required to control leakage. However, if the valve and the seal are both circular the amount of compression remains constant entirely around the valve. This contributes substantially to the high torque because of the effect of the moment arm for that portion of the valve which is substantially spaced from the axis of the stem.